This phase III trial compares the effect of radiation therapy combined with hormonal therapy versus hormonal therapy alone in treating patients with low risk, early stage breast cancer with Oncotype Dx Recurrence =< 18. Oncotype DX is a laboratory test which results in a score that is used to help predict whether breast cancer will spread to other parts of the body or come back. Radiation therapy uses high doses of radiation to kill cancer cells and shrink tumors but may result in some side effects. Hormones called estrogen and progesterone may contribute to the growth of breast tumor cells. Hormone therapy, also called endocrine therapy, may stop the growth of tumor cells by blocking or removing these hormones. This clinical trial may help researchers understand if patients with low-risk, early stage breast cancer who have Oncotype recurrence score of =< 18 can safely omit radiation therapy and only be treated with hormonal therapy without losing any radiation treatment benefit.

This is a Phase 1b open-label, 2-part study in 2 treatment groups. The 2 treatment groups are
as follows:

Treatment Group 1: OP-1250 in combination with ribociclib (KISQALI, Novartis Pharmaceuticals
Corporation).

Treatment Group 2: OP-1250 in combination with alpelisib (PIQRAY, Novartis Pharmaceuticals
Corporation).